What is MongoDB

MongoDB is a popular “NoSQL” database. The easiest way to think of it is as a way to store JSON objects in the cloud.

Why would I want/need MongoDB instead of SQL tables

If you already have the ability to create database tables based on the relational database model (e.g. with an SQL-based database such as H2/Postgres, as illustrated with the @Entity and @Repository annotations in the example Spring projects in CMPSC 156, you may wonder why a MongoDB database is used in some cases.

It isn’t just because “it’s another cool technology to learn that might be useful in a future job”, though that is also true. Rather, it’s because MongoDB is the “right tool for the job” for a particular use case that arises in some of our applications; a use case where an SQL table could be used, but would be much more challenging, time-consuming, and error prone.

That use case is when the data we want to work with is already in JSON form, such as this example of a data about a course retrieved from the UCSB Developer API:

(Note: the original object is much larger; I’ve removed some fields that don’t add anything to the explanation, and the original had four elements in the sections array.)

{
  "quarter": "20232",
  "courseId": "CMPSC   156  ",
  "title": "ADV APP PROGRAM",
  "description": "Advanced application programming using a high-level, virtual-machine-based language. Topics include generic programming, exception handling, automatic memory management, and application development, management, and maintenanc e tools, third-party library use, version control, software testing, issue tracking, code review, and working with legacy code.",
  "unitsFixed": 4,
  "classSections": [
    {
      "enrollCode": "07419",
      "section": "0100",
      "enrolledTotal": 72,
      "maxEnroll": 72,
      "timeLocations": [
        {
          "room": "1431",
          "building": "SH",
          "roomCapacity": 76,
          "days": " T R   ",
          "beginTime": "14:00",
          "endTime": "15:15"
        }
      ],
      "instructors": [
        {
          "instructor": "CONRAD P T",
          "functionCode": "Teaching and in charge"
        }
      ]
    },
    {
      "enrollCode": "07427",
      "section": "0101",
      "enrolledTotal": 24,
      "maxEnroll": 24,
      "timeLocations": [
        {
          "room": "3525",
          "building": "PHELP",
          "roomCapacity": null,
          "days": "  W    ",
          "beginTime": "17:00",
          "endTime": "17:50"
        }
      ],
      "instructors": [
        {
          "instructor": "DE A",
          "functionCode": "Teaching but not in charge"
        },
        {
          "instructor": "ROSS V E",
          "functionCode": "Teaching but not in charge"
        }
      ]
    },
  ]
}

As you can see, the overall shape of the JSON is that:

• At the top level, there is an object with some keys and values. If we only needed to store the first four values, that could easily be done with a row in an SQL database table.
• But then we encounter an array of sections. In a relational database, that would require a separate table, and a “join” relationship where we associate the sections with the top level course record. We’d have to manage the keys properly to get the correct relationship.
• Then, it gets worse. Each section can have multiple times/locations, and multiple instructors. That means a third and fourth table, and managing all of those keys.

By contrast, with MongoDB, we can just store this entire object, as is as a single object, what MongoDB calls a document.

What do I need to know to get started?

The basic unit of data in MongoDB is a document. A document is any JSON value, which can be as simple as a scalar value (e.g. true, Michelle, 24), or an array, or a deeply nested JSON object.

Documents are grouped into what are known as Collections.

It’s not a requirement, but a typical good practice is that all documents in a particular store documents with the same json “shape” (i.e. the same keys, and same nesting structure, sometimes called a JSON schema.

Where can I learn more?

There’s lots more to learn about MongoDB. Here are some resources:

• https://ucsb-cs156.github.io/topics/mongodb/
• W3 Schools MongoDB tutorial (shorter): https://www.w3schools.com/mongodb/
• MongoDB online course (longer): https://learn.mongodb.com/learning-paths/introduction-to-mongodb